group scheme of multiplicative unitsGroupSchemeOfMultiplicativeUnits2004-02-11 04:19:262005-03-25 20:39:15Exampleschemegroup scheme of multiplicative units% this is the default PlanetMath preamble. as your knowledge
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Let $R=\mathbb{Z}[X,Y]/\left<XY-1\right>$. Then $\Spec R$ is an affine scheme. The natural homomorphism $\mathbb{Z}\to R$ makes $R$ into a scheme over $\Spec \mathbb{Z}$, i.e. a $\mathbb{Z}$-scheme.
What are the $\mathbb{Z}$-points of $\Spec R$? Recall that an $S$-point of a scheme $X$ is a morphism $S\to X$; if we are working in the category of schemes over $Y$, then the morphism is expected to commute with the structure morphisms. So, here, we seek homomorphisms $\mathbb{Z}[X,Y]/\left<XY-1\right> \to \mathbb{Z}$. Such a homomorphism must take $X$ to an invertible element, and it must take $Y$ to its inverse. Therefore there are two, one taking $X$ to $1$ and one taking $X$ to $-1$. One recognizes these as the multiplicative units of $\mathbb{Z}$, and indeed if $S$ is any ring, then the $S$-points of $\Spec R$ are exactly the multiplicative units of $S$. For this reason, this scheme is often denoted $\mathbb{G}_m$. It is an example of a group scheme.
We can regard any morphism as a family of schemes, one for each fibre.
Since we have a morphism $\mathbb{G}_m \to \mathbb{Z}$, we can ask about the fibres of this morphism. If we select a point $x$ of $\Spec \mathbb{Z}$, we have two choices. Such a point must be a prime ideal of $\mathbb{Z}$, and there are two kinds: ideals generated by a prime number, and the zero ideal. If we select a point $x$ with residue field $k(x)$, then the fiber of this morphism will be $\Spec R \times \Spec k(x)$, which is the same as $\Spec R\otimes k(x)$. But looking at the definition of $R$, we see that this is $\Spec k(x)[X,Y]/\left<XY-1\right>$, which is just the scheme whose points are the nonzero elements of $k(x)$.
In other words, we have a family of schemes, one in each characteristic. Of course, normally one wants a family to have some additional sort of smoothness condition, but this demonstrates that it is quite possible to have a family of schemes in different characteristics; sometimes one can deduce the behaviour in one characteristic from the behaviour in another. This approach can be useful, for example, when dealing with Hilbert modular varieties.